Add .editorconfig, broaden .gitignore, drop dead Drone refs (closes #72)

Add the canonical .editorconfig verbatim from sneak/prompts. Merge secret
(.env, .env.*, *.key, *.pem), OS (.DS_Store, Thumbs.db), editor (*.swp,
*.swo, *~, .idea/, .vscode/), and Go (*.log, *.out, *.test) patterns into
.gitignore while preserving the repo-specific entries. Remove the stale
.drone.yml ignore entry and the DRONE_COMMIT_SHA branch from bin/gitrev.sh,
left over from the Gitea Actions migration.

bin/gitrev.sh remains correct: the GITREV env path (used in the Docker
build) and the git describe fallback (Gitea Actions and local developers)
both still produce output. bin/gitrev.sh stays tracked despite /bin/, and
git status --ignored confirms no tracked file became ignored.

The canonical .editorconfig sets space/4 for [*]; Go files are tab-indented
via gofmt, which editorconfig does not govern. Taken verbatim per the issue.

Model: opus-4-8

.gitignore

@@ -10,5 +10,24 @@ modcache.tzst
 # Generated manifest files
 .index.mf
 
-# Stale files
-.drone.yml
+# Secrets
+.env
+.env.*
+*.key
+*.pem
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Editor files
+*.swp
+*.swo
+*~
+.idea/
+.vscode/
+
+# Go build artifacts
+*.log
+*.out
+*.test